The invention concerns a device and a method of controlling the drive of a conveyor installation in the form of an escalator or a moving sidewalk, which can be switched between the load and the idle operation mode. This conveyor installation comprises a line voltage connection which supplies an essentially constant line frequency, an electric drive motor, particularly in the form of an induction or a synchronous motor, and a conveyance demand signal generator that signals a demand to switch operation modes.
A typical conveyor installation in the form of an escalator or a moving sidewalk for transporting passengers comprises a number of closely adjacent tread plates in the form of an endless band which are moved in the desired direction by the drive motor.
To reduce the power consumption and the wear of such conveyor installations, they now move only when conveyance is needed, otherwise they are brought to a standstill. To that end a conveyance demand signal generator is provided, for example in the form of a tread plate, a photosensitive relay or a manually activated switch, whereby the existence of a conveyance demand can be determined. If there is a conveyance demand, for example because a passenger has stepped on the tread plate, the conveyor installation goes into the conveyance mode for a predetermined period of time, and is then switched off if no further conveyance demand has been determined within a predetermined period of time.
To avoid peak loads during frequent on and off switching of the conveyor installation, it is known from WO 98/18711 not to switch the drive motor abruptly on and off, but to allow its RPM to increase or decrease linearly when switching. Such conveyor installations predominantly use induction motors. Since the RPM of an induction motor depends on the frequency of the supplied alternating voltage, which in the case of the direct supply from an alternating voltage network with constant line frequency means a constant RPM of the induction motor, a controllable frequency converter is used whereby the supplied line frequency can be converted in a controllable manner into an output frequency which differs from the line frequency.
The cost of a frequency converter which also supplies the drive motor of an escalator or a moving sidewalk in the load operation mode is high, since it increases enormously with the output power that a frequency converter must be able to produce.
To lower the acquisition and operation costs, WO 98/18711 provides that the conveyor installation only moves at full conveyance speed in the load operation mode, and that in the stand-by or idle mode when no conveyance is required, it only operates at a reduced idle operation speed, and the frequency converter only supplies the drive motor during idle mode and switch-over processes, while it is directly supplied by the line voltage source in the load operation mode. This creates the possibility of designing the maximum output of the frequency converter to be much lower, which leads to considerable cost savings as compared to a frequency converter whose maximum output is adapted to the conveyor installation""s load operation. If no further conveyance demand is signalled after a conveyance order has been carried out, the conveyor installation in WO 98/18711 first changes into the idle mode, and only goes into standstill if no new conveyance demand is signalled during a predetermined period of time since the change-over to the idle mode.
The cited measures have achieved a considerable reduction in load peaks and abrupt speed changes of the conveyor installation. However, high transition currents can always occur when changing between the line supply and the frequency converter supply of the drive motor, namely due to the drive motor""s own characteristic voltage which can overload the frequency converter and cause jerky movements of the conveyor installation. The present invention will overcome such events.
This is achieved with a method according to the invention as claimed in claim 1, and a device according to the invention as claimed in claim 11, where further developments of the method or the device are indicated in the dependent claims.
The method of the invention as well, the drive motor in the load operation mode is supplied with a line voltage, and in the idle mode with an output voltage of a frequency converter. To achieve the target of the invention, the line voltage and the frequency converter""s output voltage are compared with respect to frequency and phase position, and the frequency converter is set to an output frequency which has a predetermined spacing from the line frequency. If a conveyance signal generator signals a demand for a switch-over of the conveyor installation from load to idle mode or vice versa, at the point in time after the demand to change the mode was signalled, when the output frequency of the converter has the same spacing with respect to the line frequency and has also reached a predetermined phase spacing between the output frequency of the converter and the line frequency, a signal is produced which triggers the switch-over of the drive motor between the frequency converter supply and the line supply.
The switching devices used to switch between the line supply and the frequency converter supply, usually contactors, are not delay-free on the one hand, and on the other require a zero current time between the turn-off of one contactor and the activation of the other in order to prevent a short circuit in the line through the frequency converter. There is a certain inherent reaction delay between the production of a switch-over signal and the previously conducting contactor turning off, and finally the activation of the other contactor, which depends on the special components of the special conveyor installation.
A smooth transition between the frequency converter supply and the line supply and vice versa can therefore not be achieved by monitoring the line voltage and the frequency converter""s output voltage for a match in the frequency and phase position, and by producing a switch-over signal at the time when such a match is determined. By the time the inherent reaction delay actually produces a switch-over, a frequency and phase deviation would already have occurred and no smooth transition could take place between the line supply and the frequency converter supply.
The invention therefore produces a switch-over process xe2x80x9cin advancexe2x80x9d, i.e. it plans the reaction delay and the frequency and phase changes that occur during the reaction delay between the line frequency and the output frequency of the converter. To that end it determines in a special conveyor installation the inherent reaction delay, the frequency change in the motor voltage which occurs during the reaction delay, and the change in the phase difference between the line frequency and the output frequency of the converter which occurs during the reaction delay, and produces the switch-over control signal at a time in advance, when a predetermined frequency spacing as well as a predetermined phase spacing exist between the line voltage and the frequency converter""s output voltage. The frequency and phase match needed for a smooth switch-over is then available at the end of the reaction delay.
The sign of the predetermined frequency spacing depends on the switch-over direction. Due to inherent friction losses in the conveyor installation, the motor RPM decreases during the zero current time when the drive motor is not supplied by either the line or the frequency converter. Since we start with a constant line frequency, which the motor RPM must match at the end of a switch-over process, the predetermined frequency spacing of the converter""s output voltage is above the line frequency when the frequency converter supply is switched over, and it is under the line frequency when the line supply is switched over to the frequency converter supply.
The method of the invention can be carried out with an electric controller for controlling the drive of a conveyor installation in the form of an escalator or a moving sidewalk that can be switched between a load and an idle operation mode, which has a line voltage connection with an essentially constant line frequency, a drive motor and a conveyance signal generator which signals a mode switch-over demand, where the controller has a frequency converter whose output frequency can be set, and a controllable switch-over installation with a load operation circuit where the drive motor is directly coupled to the line voltage connection, and an idle mode circuit where the drive motor is coupled to the line voltage connection via the frequency converter, and a synchronizer whereby the converter""s output frequency can be set, where the output voltages of the line voltage connection and frequency converter are comparable with respect to frequency and phase position, the frequency converter can be set to an output frequency with a predetermined spacing from the line frequency, and a signal can be sent to the switch-over installation after a mode switching demand has been signalled, when the converter""s output frequency has both the predetermined spacing to the line frequency, and has reached a predetermined phase spacing between the output voltages of the frequency converter and the line voltage connection.
In a preferred configuration of the invention, each of the switching devices in the switch-over installation is a contactor. Contactors that are designed for a switching capacity, as needed in conjunction with escalators or moving sidewalks, usually have an inherent switch-off delay between the reception of a switch-off signal and the actual transition into the non-conducting mode, and an inherent activation delay between the reception of a switch-on signal and the actual transition into the conducting mode. In this case the inherent reaction delay of the switch-over installation is composed of the delay in releasing the until now conducting contactor, the zero current time during which no power is supplied to the drive motor, and the activation delay of the until now non-conducting contactor.
The predetermined frequency spacing and the predetermined phase spacing of line voltage and frequency converter""s output voltage, which must occur at the time when the switch-over signal is produced, are empirically determined on the basis of the respective zero current time, and possibly the respective switch-off delay in a practical conveyor installation configuration. To that end it is determined to what degree the frequency and phase position of the drive motor""s terminal voltage changes during the zero current time, and how long the turn-off delay is. The result is the point in time when the switch-over signal must be produced so that at the time when the switching installation becomes conductive, there is at least an essential match with repect to frequency and phase position between the voltage at the motor terminal and the motor supply source connected to this switching installation.
In order to take the decrease in the drive motor""s RPM during the respective zero current time into consideration, the converter""s output frequency is set for a predetermined frequency spacing above or below the line frequency, depending on whether the conveyor installation operates in the load or in the idle mode. In both cases the frequency spacing is chosen so that it corresponds to the decrease in the frequency of the motor terminal voltage during the zero current time of the respective switch-over process.
In conjunction with the switch-over processes, one configuration of the invention provides for the converter""s output frequency to be set with a rising or a declining slope. Outside of such slopes the converter""s output frequency is preferably set for the predetermined frequency spacing as opposed to the line frequency. A two-point adjustment can be used to that end. To rapidly reach the final load RPM of the drive motor on the one hand, and to obtain a short adjusting process when switching over from the standstill to the load operation on the other hand, one configuration provides that the converter""s output frequency is set with a steeper slope on a first ramp part that lies at a predetermined spacing under the line frequency, and it is set with a flatter slope when it reaches this spacing.
One controller of the invention has a switch-over installation for switching between the load and the idle mode, which preferably contains two contactors, and a synchronizer whereby the converter""s output frequency can be set, where the line voltage and converter""s output frequencies are comparable with respect to frequency and phase position, where the frequency converter can be set for the predetermined spacing from the line frequency, and a switch-over signal can be sent at the time after a demand to switch modes has been signalled, when the frequency converter""s output frequency has both the predetermined spacing to the line frequency, and a predetermined phase spacing has been achieved between the output voltages of the frequency converter and the line voltage.